1. Field of the Invention
The present invention relates to a piezoelectric ceramic transformer, which is operable at a high frequency band and more particularly to a piezoelectric ceramic transformer for an onboard power supply whose smaller size and lower noise are demanded.
2. Description of Prior Art
Many efforts have been made to make the switching frequency of a switching power source higher for providing more compact electronic equipment. An electromagnetic transformer has heretofore been used for the switching power source. As the switching frequency is increased, the power loss due to hysteresis loss and eddy current loss of the magnetic material used in the electromagnetic transformer and the loss due to the skin effect of the conductor are sharply increased, resulting in a considerably low efficiency of the transformer. Accordingly, the upper limit of the practical frequency of the electromagnetic transformer is at most 500 kHz.
In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer is used in a resonant mode and has a number of advantages as follows:
(1) The size of the transformer can be made smaller since the energy density at the same frequency is higher, PA1 (2) The transformer can be made nonflammable. PA1 (3) No noise due to electromagnetic induction is generated.
The structure of a Rosen type piezoelectric transformer which is a typical prior art piezoelectric transformer will be described with reference to FIG. 1 as follows. In a piezoelectric ceramic plate for outputting a high voltage which is provided with electrodes on surfaces thereof, a reference numeral 41 denotes a low impedance drive portion of the piezoelectric transformer, which is provided with electrodes 43 and 44 on the upper and lower surface thereof, respectively. A reference numeral 42 denotes a high impedance generating portion which is provided with an electrode 45 on the end side thereof and the portion 42 is polarized in a longitudinal direction of the piezoelectric ceramic plate as represented by a reference numeral 47. The piezoelectric transformer functions as follows: When a voltage is applied across the driving electrodes 43 and 44, length vibration is excited at an electromechanical coupling factor k.sub.31 in a transverse effect 31 mode so that the whole of the transformer vibrates. In the generating portion 42, a high voltage can be outputted from the output electrode 45 in a longitudinal vibration 33 mode due to longitudinal effect at an electromechanical coupling factor k.sub.33.
On the other hand, in the case that it is desired to output a low voltage by inputting a high voltage, it is apparent that it will be sufficient to use the longitudinal effect high impedance portion 42 and the transverse effect low impedance portion 41 as input and output sides, respectively. Any of the other piezoelectric transformers utilizes elongation vibration of a flat plate which is same as that of the Rosen type piezoelectric transformer and radial expanding vibration of a disc in a radial direction and thus has an applicable frequency which is up to 200 kHz.
The present inventors have proposed a piezoelectric ceramic transformer having a structure in which piezoelectric ceramic plates which have been polarized in a thickness direction are stacked and which is capable of operating in the order of MHz band by being driven at a resonant frequency in a thickness extensional vibration (refer to U.S. patent application Ser. No. 530,930, filed on May 30, 1990 ).
When the multilayer piezoelectric ceramic transformer using a thickness extensional vibration mode which has been already proposed by the present inventors is driven, if the polarization of the low impedance portion is uniform in a thickness direction and has the same vector of polarization and the thickness of each ceramic layer is uniform, cancellation of electric charges occurs in the whole of the low impedance portion resulting in that electric fields induced by the ceramic layers are cancelled with each other. Briefly, the stored electrostatic energy is cancelled, resulting in a decrease in electric power transmission efficiency.
In the prior art polarization method, the whole of the element was uniformely polarized in a thickness direction and thereafter the ceramic layers of the low impedance portion were polarized in opposite directions with each other. In this method, delamination of electrodes or breaking of the piezoelectric ceramic layers partially occurs due to mechanical strain associated with reversal of polarization in the low impedance portion. Thus, there is a disadvantage that the mechanical quality coefficient Qm of the piezoelectric ceramic transformer lowers and the electric power transmission efficiency of the piezoelectric ceramic transformer is low.
It has been also difficult to provide the proposed piezoelectric transformer with a plurality of pairs of output terminals in view of its structure.